This invention relates to a method for making carbon-encapsulated ultrafine metal particles by the application of plasma assisted evaporation.
There has been a need, so far unresolved, for a method for making carbon-encapsulated ultrafine metal particles. Such materials, with a particle diameter less than 1000 Angstroms, exhibit properties which make them valuable for many applications, including utilization of a wide range of air-sensitive metals, formation of catalysts with enhanced activities at relatively low temperatures, and making of solid lubricants pigments and toners. Accordingly, the mass-production of carbon-encapsulated ultrafine metal particles at low cost is widely desired.
The high temperature catalytic reduction of carbon monoxide on the surface of iron particles is a well-known method for making ultrafine carbon-coated iron particles. This technique however, is not suitable for mass production because the formation rate of the particles is slow and the production rate of the particles is low.
The most successful among the known methods for making carbon-encapsulated ultrafine metal particles with an improved formation rate is the high current carbon arc evaporation process.
In the article entitled, "Single Metals Encapsulated in Carbon Nanoparticles", Science, Vol. 259, 15 Jan. 1993, (Rodney S. Ruoff, Donald C. Lorents, Bryan Chan, Ripudaman Maltotra, Shekhar Subramoney) there is disclosed a technique for making carbon-encapsulated ultrafine metal particles of .alpha.-LaC.sub.2 by this process.
Similar technological method has been disclosed in "Cobalt Catalyzed Growth with Single Atomic Layer Walls", Nature, Vol 363, 17 Jun. 1993, (D. S. Bethune, C. H. Kiang, M. S. de Vries, G. Gorman, R. Savoy, J. Vazques and R. Beyers) for producing graphite-encapsulated nanocrystals of magnetic materials (Fe, Ni or Co). It is this technique in which metal powder is packed within a recess that is drilled in the positive graphite electrode. The arc conditions included a D.C. current of 150A, a gap distance of about 1 mm between negative electrode and translating positive electrode, and He pressure of 500 torr. This technique utilizes plasma as a vaporization heater and it is more productive than the high temperature carbon monoxide catalytic reduction method. However this technique still produces a relatively small quantity of product because while all the metal powder is vaporized only a small amount which fits in the cathode cavity can produce product at one time. Thus, it is difficult to employ this technique on a profitable basis.
Therefore, it has been a goal in the art to produce a higher yield, commercially viable process of producing carbon encapsulated ultrafine particles.